The process of machining a series of spaced concentric bores with the same diameter where the distance between adjacent bores is not too large (less than 1-5 bore diameters) is called line boring. Line boring requires use of dedicated manufacturing equipment and is a very demanding application in terms of both quality and production rate requirements.
In typical automotive applications the range of machined bore diameters is 25-75 millimeters (1-3 inches); the required cylindricity of the bore is 10-25 microns, and the surface finish R.sub.a is 2-50 microns. In extreme cases line boring bars can be as long as 1350 millimeters (53 inches) and weight up to 120 kg (250 lb).
Boring tools are recognized as a major bottleneck in increasing the flexibility and precision of the line boring processes. One of the major obstacles to increasing flexibility of the boring tool is that current tooling designs require outboard and intermediate support bushings. If these bushings are eliminated, automated tool change can be accomplished more readily, and system reliability is increased through simplified design and the elimination of bearing support components that wear. However, elimination of supports makes the boring bar more compliant thus leading to increased vibration and decreased precision. In a typical line boring station, the spindle is mounted horizontally, and a single dedicated boring bar which is responsible for machining the bore through the entire length of the engine casting is mounted on the spindle. Due to the length of the bore, the boring bar is usually supported by bushings at a number of points to ensure high precision of the operation. To facilitate entry and exit of the boring bar into the bore, the fixture is provided with a raising and lowering motion. The boring operation is performed sequentially, under control of a hard-wired system.
According to the prior art, line boring bars were supported at both ends to obtain adequate rigidity. As a result, all the journals could be machined simultaneously by including multiple cutting inserts down the length of the boring bar. The next evolution was a boring bar with longitudinal carbide guide pads that are ground to a diameter slightly smaller than the desired final bore diameter. In this process, a single-point dedicated tool bores the first hole, and is subsequently fed into the part until all journals are completed. The need for the outboard support is eliminated by using the part itself to support the tool with the guide pads as subsequent journals are machined. Because the length of the boring tool projecting from the spindle is constant (the part or the spindle itself moves), the deflections of the boring bar are also expected to be constant and the holes will be in precise alignment (assuming adequate rigidity of the part and fixturing). This is shown in FIG. 1.
The newest line boring development being used in manufacturing today is shown in FIG. 2 and is an extension of the guide pad concept. This method moves the tool in the feed direction relative to the spindle.
Mapal Inc. has developed fineboring bars that feature an adjustable, indexable blade on each tool and at least two guide pads on the periphery of the tool to support the tool immediately after a cut. Such a design increases the stiffness and the first natural frequency of the bar which leads to improved boring precision. A Danish patent by Johne, DK 3,926,025 [1991] shows a machine tool precision boring head which uses flexible parallel springs to serve as an end support. A Japanese patent by Tajima, JP 63185505A [1988] enhances productivity and machining accuracy by automatically exchanging a line boring bar and support bushings and by automatically adjusting the position of supports, the boring bar and a workpiece with the use of an NC machine tool.
It is accordingly an object of the invention to provide a rotary tool with a means for adjusting the position of the cutting tool insert while the tool is rotating.
It is another object of the invention to provide a rotary tool with a means for detecting displacement of the tool from its intended position.
It is another object of the invention to provide a rotary tool with a cutting tool insert in which an electromechanical actuator is used to adjust the position of the insert relative to the tool.
It is another object of the invention to provide a rotary boring tool in which the position of a cutting tool insert is compensated for unintended displacement due to workpiece engagement or boring tool droop.
It is another object of the invention to develop signals used to correct the position of a cutting tool insert in a rotating tool by means of a computer and other electronics that rotate with the tool.